A. Field of the Invention
This invention relates to microwave mixer circuits which incorporate passive inductive elements as negative feedback components to provide noise reduction and a wide dynamic range for the mixer. More specifically, the present invention uses degenerative inductors in conjunction with the input transistors to maintain linearity of the circuit while reducing problems due to low voltage and thermal noise as discussed hereinafter. In a balanced configuration, the mixer may also include an inductor or a parallel inductor-capacitor resonant circuit to act as a constant current source at RF frequencies to maintain symmetry of the signal from the input stage.
B. Description of Related Art
Various forms of signal multipliers have been known for processing of signals in a wide variety of applications, e.g. mixer and modulator circuits. A two-signal four-quadrant multiplier commonly known as a "Gilbert cell mixer" was described by Gilbert, A Precise Four-Quadrant Multiplier with Subnanosecond Response, IEEE Journal of Solid-State Circuits, Vol. SC-3, pp. 365-373 (Dec. 1968). As described in more detail herein, these circuits typically are composed of two pairs of switching transistors which are cross connected, and which switch the outputs of a further pair of signal input transistors, connected as a balanced amplifier. Mixers using Gilbert cell structures have the advantage of high isolation and positive conversion gain due to the balanced structure of the differential amplifier configuration.
In the conventional Gilbert mixer, degenerative resistive feedback is used with the input transistors to enhance the linearity of the input stage. It has been conventional to use resistors in the circuit, in part, because resistors occupy a smaller area on the integrated circuit than other components may occupy. However, the degenerative resistors generate thermal noise which can substantially degrade the noise figure of the mixer. In addition, the dc voltage drop across the resistors reduces the effective power supply voltage for the transistors in the mixer. This can be offset by an increase in the output voltage of the power source, but such an increase is costly to obtain in battery-powered units. In such cases, the input voltage must be unduly constrained in order to maintain operation of the input transistors in their linear regions. The low voltage can also degrade the operation of the switching transistors.
Another feature of the balanced mixer circuit is the use of a constant current source to maintain a constant total current through the two input transistors. This maintains symmetry in the outputs of these transistors. In the conventional mixer, the constant current source is typically a transistor. The resulting circuit configuration is a three-level stack of transistors. The problem with such a design is that the voltage drop across the constant current source reduces the supply voltage available for the input and switching stages. Again, this is a problem when a lowvoltage power supply is desired.
Other types of modulating circuits for various applications have been known. These applications include modulator-type circuits for generation of amplitude-modulated signals. In addition to AM signal generation, these circuits can be used for automatic gain control, frequency multiplication, phase detection, synchronous AM or FM demodulation, and frequency discrimination. Balanced modulators in general were described in Grebene, Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, pages 469-79 (1984).
None of the aforementioned prior circuits adequately maintain a wide operating range at low power supply voltages. There remains a need, therefore, for a circuit which has a wide operating range even in the presence of a low supply voltage, such as 3.6 volts or lower, and which circuit includes degenerative feedback providing increased linearity of the circuit. Furthermore, there remains a need for a circuit which has a constant current source but which current source does not promote a further drop in the available voltage swing for the input transistors and the switching transistors of the circuit.
Accordingly, it is an object of the present invention to provide a circuit which has a wide dynamic range at low power supply voltages, for example. It is a further object of the invention to use degenerative elements which improve the linearity of the circuit but do not add to a substantial dc voltage drop, thus decreasing the obtainable voltage swing across the transistors of the circuit. It is also an object of the invention to do so with the addition of minimum noise as a low-noise requirement is particularly important in the processing of comparatively weak microwave signals.
It is yet another object of the invention to provide the input transistors with a constant current source which has a negligible dc voltage drop across it. Yet another object of the invention is to meet these needs while allowing for convenient adaptability within microwave integrated circuits and remaining within the space limitations encountered in such integrated circuit technology.